The invention relates to error correction code strategies for parallel track recording.
Parallel track recording media, magnetic tape in particular, is vulnerable to a number of error producing physical defects capable of producing "hard errors" or "dropouts" which can effectively destroy prerecorded data at specific locations. Recording densities in terms of "bits per inch", for example, for high performance digital magnetic tape are increasing rapidly. As the memory cells on the tape become more closely packed, the magnetic energy available for "reading" the stored data becomes substantially lower. The higher the density, the lower the read signal level. As a result, the trend toward higher densities in itself makes the medium more susceptible to dropouts.
One way to cope with the growing dropout problem is to employ "write margining" designed to make the recording less vulnerable to errors which arise during the write mode. These techniques, of course, will not help if the tape gets damaged at a later time prior to rereading the information. Self erasure and tracking problems pose additional risks exacerbated by rising storage densities.
Storage systems designers embed special code words in the prerecorded data which can be used not only to check the validity of the data but also to reconstruct missing portions. Data is typically stored in a series of equal length blocks. Each block consists of a series of binary digits and has its own data checker (e.g., check sum or cyclical redundancy check (CRC code) associated with the block. In addition, the data in a series of blocks on a given track can be linked to an error correction code (ECC) stored in another block either on the same track or on another track. Because the ECC is an algebraic combination of the data in the series of blocks, a single, entirely missing data block can be reconstructed algebraically by using the ECC, assuming that the ECC is recovered intact. ECC strategies presently in use do not deal adequately with the higher error rates exhibited by high density tape systems presently under development.